docs: def train_crf(ctx, input, output, clusters):
      """Train CRF CEM recognizer."""
      click.echo('chemdataextractor.crf.train')
      sentences = []
      for line in input:
          sentence = []
          for t in line.split():
              token, tag, iob = t.rsplit('/', 2)
              sentence.append(((token, tag), iob))
          if sentence:
              sentences.append(sentence)
  
      tagger = CrfCemTagger(clusters=clusters)
      tagger.train(sentences, output)